1. Field of the Invention
The present invention generally relates to milling machines and their use in milling nonferrous metals, such as aluminum. More specifically, this invention relates to a face milling machine whose construction and operating parameters provide for an adiabatic very high speed machining process with improved tool life and operating efficiencies, wherein the chips are evacuated from the workpiece and the cutter without the use of cooling liquids or lubricants. In addition, the milling machine includes accessories that further promote the adiabatic operation, precision and efficiency of the milling operation.
2. Description of the Prior Art
Milling machines are widely used in manufacturing processes for producing close tolerance parts, from flat surfaces to slots, keyways and various complex contours. In particular, face milling machines are employed where planar surfaces are to be machined to flatness tolerances of 0.003 inches (0.076 millimeters) and less. Face milling machines typically include a spindle which is rotatably held perpendicular to the surface of a workpiece. The cutting element, or cutter, is generally a disc mounted to the end of the spindle. The cutter has a number of teeth formed on, or alternatively, a number of cutting inserts mounted at, its perimeter, such that the outside diameter of the cutter removes the stock from the workpiece being machined. The cutter is rotated by the spindle, which in turn is driven by a motor of suitable horsepower. Cooling liquids are commonly used to lubricate, cool and flush chips from the workpiece and cutter. Finally, the workpiece and spindle are moved relative to each other to feed the workpiece into the cutter, denoted as the feed rate and traditionally measured in inches per minute. Alternatively, feed rates are provided in inches per tooth, given by the formula: EQU FR/((RPM)(t));
where FR is the feed rate (the rate of relative movement between the workpiece and the spindle) in inches per minute, RPM is the rotational speed of the spindle in revolutions per minute, and t is the number of cutting teeth or inserts on the cutter.
Cutting speed, and its relation to feed rates, is of primary importance if a milling machine is to efficiently produce close tolerance, high surface quality parts. In the past 25 years, particular attention has been concentrated on cutting speed and its effects on the quality and efficiency of the milling process. Cutting speeds are indicated in surface feet per minute (sfm) which can be calculated by the following formula: EQU 2.pi.(r)(RPM);
where r is the radial dimension of the cutting teeth from the spindle's axis of rotation in feet, and RPM is the rotational speed of the spindle in revolutions per minute. Appropriate cutting speeds are dependent upon several factors--primarily the material being cut and the material of the teeth or cutting inserts used, with nonferrous metals, such as aluminum, and carbide cutting inserts usually allowing for higher cutter speeds.
No one classification of cutting speeds has been generally accepted, but the 16th Volume of the Metals Handbook (9th Edition) entitled "Machining" and published by the American Society of Metals, suggests that cutting speeds can be classified as follows. Conventional cutting speeds are below 2000 sfm for nonferrous metals, and often less than 500 sfm for ferrous metals. Higher speeds of 2000 to 6000 sfm are deemed high speed machining, speeds of 6000 to 60,000 sfm are deemed very high speed machining, and speeds greater than 60,000 sfm are ultrahigh speed machining. Obviously, one advantage to higher machining speeds is faster machining time and thus higher production rates. A significant additional benefit to high speed machining is that, past a critical cutting speed which is characteristic of the particular material being machined, cutting forces actually decrease with increased spindle speed until a minimum is reached, which is again a characteristic of the given workpiece material. Accordingly, cutting forces at higher speeds can actually be comparable to or less than that at conventional speeds. Low cutting forces are not only desirable from the standpoint of the power requirement of the spindle's motor, but are particularly desirable when machining very thin, nonrigid workpieces.
Finally, an additional benefit to high speed machining is the ability to achieve a substantially adiabatic cutting operation in which nearly all of the heat generated during the machining process is transferred to the chips formed, thus keeping the cutter and the workpiece essentially at their original pre-machining temperatures. In addition to being able to handle the workpieces immediately after machining, other significant advantages to achieving an adiabatic operation are improved cutting efficiency, less spindle power, lower noise levels, higher precision cuts, less workpiece deflection, and improved tool life. Again, such advantages are particularly beneficial when machining thin, nonrigid workpieces.
Moreover, a coolant is not always needed under adiabatic machining conditions, and in fact may adversely serve to transfer heat from the chips back to the workpiece and cutter. Though cooling liquids generally improve tool life and the appearance of the machined surface, they require extensive delivery, filtering and often cooling systems. Also, the use and disposal of cooling liquids are a growing health and environmental concern. Accordingly, dry machining provides several significant advantages over the use of coolants.
However, to sustain a truly adiabatic cutting operation, particular attention must be given to the type of material being cut and the material of the teeth or cutting insert, the appropriate feed, speed and depth of cut, the precision by which the spindle is supported relative to the workpiece, the stiffness of the cutter, and the ability of the fixturing to rigidly and accurately support the workpiece.
To date, practically all scientific investigation in the area of high speed adiabatic machining of aluminum has been limited to small end mills (0.5 to 1 inch in diameter) at speeds from approximately 10,000 to approximately 60,000 rpm--or roughly 2600 to 15,700 sfm. In practice, such high rotational speeds are severely limited by bearing size, with smaller bearings allowing higher rotational speeds. However, smaller bearings simultaneously limit spindle power and stiffness, resulting in cutting speed being inversely proportional to power and stiffness. Consequently, cutting forces and horsepower limitations have effectively constrained testing to much lower speeds--typically, below 5000 sfm--for purposes of developing milling machines which are practical for use in production manufacturing. Simultaneously, stiffness of the spindle and the manner in which the cutter is mounted to the spindle has also limited cutter size, significantly limiting material removal rates.
In terms of cutting efficiency or unit power (cubic inches per minute per horsepower), the industry has generally concluded from testing thus far that, though cutting forces and specific power are reduced at higher speeds, these advantages tend to diminish above speeds of 5000 sfm. Maximum unit power for machining aluminum is generally believed to be approximately 3 and as much as 4 cubic inches per minute per horsepower at about 5000 sfm, with horsepower available from current motor technology being limited to approximately 30 horsepower at these high spindle speeds. Accordingly, to achieve higher material removal rates in excess of 40 cubic inches per minute generally requires higher-torque drive motors which result in lower cutting speeds, defeating the advantages of high speed cutting.
Moreover, cutting tool manufacturers do not recommend using cutting speeds in excess of 3000 sfm for aluminum cutting with diamond under realistic production manufacturing conditions, though a few recognize speeds as high as 12,000 sfm as being viable. However, such higher speeds have generally been limited to carbide and diamond cutting tools. Diamond cutting tools, such as polycrystalline diamond (PCD)-tipped carbides, have recently become popular for cutting aluminum because of improved tool life--by a factor of 10 to 100 over tungsten carbide cutting tools. However, diamond cutting tools are relatively brittle and are therefore limited by the ability of the milling machine's stiffness and workpiece stability to avoid impact loads caused by workpiece and cutter vibration, particularly at higher cutting speeds. Accordingly, diamond tool manufacturers currently recommend maximum cutting speeds of 1500 to 2500 sfm.
From the above discussion, it can be readily appreciated that the prior art testing does not suggest or support advantages to machining aluminum at speeds in excess of 5000 sfm. Generally, the limitations of high speed milling include spindle stiffness, excessive horsepower requirements, and cutting tool limitations, spindle/cutting tool interface limitations, and machine feed rate capability. Accordingly, high speed milling has not been widely employed under typical manufacturing conditions, even where there is a need to surface machine thin workpieces. As a result, the industry conventionally has turned to grinding for such applications. However, even where the above limitations have been achieved under strict laboratory test conditions, the prior art has failed to achieve high material removal rates, particularly in terms of specific power (i.e. cubic inches per minute per horsepower).
Accordingly, what is needed is a cost-efficient adiabatic milling machine capable of operating without cooling liquids at very high speeds, while affording improved tool life and material removal rates and surface finish, and which is particularly adapted for precision milling thin aluminum workpieces in production manufacturing.